1. Field of the Invention
The present invention relates to an image forming apparatus, particularly with a detachably mountable cartridge of an electrophotographic system such as a laser beam printer, a copying machine, a facsimile, or a multi-function printer in a combination of these. Further, the present invention relates to a method of detecting the remaining amount of developer contained in a developing device that develops an electrostatic latent image formed on an electrophotographic photosensitive member through such image forming apparatus.
2. Description of the Related Art
An image forming apparatus for forming an image based on an image signal sent from a host computer includes an electrophotographic system, an inkjet system, a thermal sensitive system, and other various systems. Among these, the image forming apparatus of electrophotographic system is becoming increasingly widespread in recent years in terms of high-speed, high image quality, and high degree of silence and other advantages.
<Regarding Process Cartridge>
Since each of components of the image forming apparatus of electrophotographic system wears and is abraded or becomes exhausted each time an image formation is executed, maintenance will be needed after a fixed amount of image formations have been executed. In particular, since a toner is consumed by the image formation, the need for replacement arises.
As a consequence, in order to replace these consumables as easily as possible, there is a system configured for a photosensitive drum unit and a toner cartridge to be replaceable independently. In addition, a process cartridge system may be adopted in some cases, in which a photosensitive drum, a charging roller, a developing device, and a cleaning blade are integrated compactly.
The former is mainly used for a copying machine or the like; when a photosensitive drum reaches its lifetime, only a photosensitive drum unit is replaced; when a toner is totally consumed, only a toner cartridge is replaced. Each lifetime is alerted to a user by issuing a warning, for example, on a display panel of the image forming apparatus by means of each lifetime detecting unit. With this method, a photosensitive drum, toner can be independently replaced, and thus it is economical. When replacing a toner cartridge, however, it is necessary for a user to do work carefully so as not to touch the toner.
On the other hand, in the latter process cartridge system, when toner is totally consumed, it is replaced together with a process cartridge. As a result, it has a merit of easy-to-handle. Further, maintenance of the apparatus can be performed by users per se without relying on servicepersons, and consequently the operability can be substantially enhanced. In recent years, widespread use centering around a small-sized image forming apparatus including a laser beam printer is being achieved.
In such a process cartridge, in order to prevent a poor image due to the exhaustion of toner, some units often detect information on the remaining amount of toner, and send a notice of the result to the image forming apparatus or a host computer to which the apparatus is connected, and then alert to users.
<Regarding the Detection of Remaining Amount of Toner>
There are various systems as a detecting unit for the remaining amount of toner. For example, there are proposed a method of detecting plural levels of the remaining amounts of toner by means of piezoelectric elements or photocouplers (Refer to Japanese Patent Application Laid-Open No. 61-176962), and a method of detecting successively the amounts of toner by measuring capacitances between metal plate-shaped electrodes (U.S. Pat. No. 5,987,269).
Further, there is proposed a method of successively detecting the amount of toner by measuring capacitances between a metal plate-shaped electrode member disposed within the image forming apparatus or within the process cartridge, and a developing member serving as a developer carrying member for conveying toner to a developing portion on a photosensitive member (Refer to Japanese Patent Application Laid-Open No. 2000-206774).
In addition to above, there are an optical system for detecting the amount of light that passes through the interior of a toner container containing toner, and a pixel signal totalizing system in which individual image signals forming dots are counted, and the amount of consumption of toner is determined by multiplying the number of counts with a predetermined coefficient.
The amount of toner detected by such a developer remaining amount detecting device is indicated on an indicating unit so that users can recognize. This recognition by users enables more efficient printing work and effective use of the process cartridge.
Among the above methods, a method of detecting the remaining amount of toner by measuring capacitances between electrode members is widely used for the process cartridge because added circuit is relatively simple, and a high accuracy can be achieved.
In a method of measuring capacitances between the above-mentioned electrode members, a metal plate (electrode metal plate at output side), for example, serving as an electrode member at output side is provided at a spot opposite to the developing member, and a metal plate (electrode metal plate at input side) serving as an electrode member at input side is provided at a spot opposite to an electrode metal plate at output side. And, each of the electrode metal plate at output side, the electrode metal plate at input side, the developing member acting as the electrode member at input side is used as an electrode for a capacitor, taking an advantage of variation of the capacitance between respective electrode members corresponding to the amount of toner.
In other words, when a space (this equates to capacitor) configured by metal plates (the electrode metal plate at output side, the electrode metal plate at input side) and the developing member is filled with toner, capacitance becomes large, whilst air within the above space increases and capacitance becomes small as the amount of toner decreases. The remaining amount of toner is detected by utilizing the variations of these capacitances.
If the sizes, areas of metal plates as electrode members, or distances between the metal plates are different, detected capacitance values introduce variations reflecting these individual differences. If the metal plates are provided within the process cartridge, it is difficult to avoid introducing individual differences in the distance between the metal plate and the developing member, the distance between metal plates, and the size of metal plates from a manufacture viewpoint. For this reason, if the remaining amount of toner is detected using a relationship that has been set in advance between a specific capacitance and the remaining amount of toner, it is difficult to detect accurate amount of the remaining toner in conformity with respective process cartridges due to individual differences like the above.
For this reason, the following methods are proposed in order to perform accurate toner remaining amount detection in conformity with respective process cartridges regardless of individual differences like the above.
More specifically, a method of storing an output value when a maximum amount of toner, i.e. a maximum capacitance is detected by a developer remaining amount detecting unit, and detecting the remaining amount of toner based on its reference value (Refer to Japanese Patent Application Laid-Open No. 2001-134064) is proposed.
Further, there is proposed a method of setting a threshold value for an output value as a reference value on a storage medium when a maximum amount of toner, i.e. a maximum capacitance is detected by the developer remaining amount detecting unit, and coping with a deviation of the output value serving as a reference (Refer to Japanese Patent Application Laid-Open No. 2002-268362).
A method of setting a detected value corresponding to a maximum capacitance value as a reference value as described above, detecting an amount of change between the reference value and an output value at this time point, and estimating the remaining amount of toner is used widely because of high accuracy and less-complex processing.
However, a conventional method of detecting the remaining amount of toner by measuring capacitances between electrode members, as described above, has the following problems.
In other words, in the case where an use environment of the process cartridge changes, or in the case where a toner condition changes accompanied by a long-term use, capacitance values detected by the developer remaining amount detecting unit have fluctuated during use of the process cartridge. Therefore, there was a limit in measuring accurately the remaining amount of toner.
For example, water has a higher relative permittivity than air, and thus the more water is present between electrode members, the larger capacitance is detected. Hence, a larger capacitance is detected under a high humidity environment than under a low humidity environment, and the accuracy in detecting the remaining amount of toner becomes worse if use environment of the process cartridge extends over both high humidity and low humidity. In particular, if there is a difference between an environment under which the amount of toner that determines a reference value is detected at maximum, and an environment under which the remaining amount of toner becomes small, a detection error becomes large.
Also, this is the same with the case where the condition of toner has changed. For example, if a printing ratio at which users perform printing is extremely low, there is a possibility that the use of the process cartridge extends over a longer period, and toner gets deteriorated. Deteriorated toner tends to have a larger density compared with fresh toner, and in this case, even for a toner with the same volume, its capacitance value will become small due to its low density.
Also, detected capacitance value fluctuates even with adherence of some amount of toner to an electrode metal plate. The amount of toner that has adhered to the electrode metal plate varies depending on each cartridge, depending on each condition in which it has been used. The more the amount of toner has adhered, the more measured capacitances are detected than actual amount of toner.
As described above, there was a possibility that the accuracy in detecting the remaining amount of toner becomes worse due to various factors.
In an apparatus that makes an erroneous detection in which detected amount of toner tends to be smaller than an actual amount of toner, the followings are considered.
For example, a user probably replaces a process cartridge with new one according to indicated remaining amount of toner that is related to the output value of the developer remaining amount detecting unit, in spite of some toner remaining in reality. This is undesired from viewpoint of effective utilization of resources too.
Also, on the contrary, in an apparatus that makes an erroneous detection in which detected amount of toner tends to be larger than an actual amount of toner, for example, toner is totally exhausted during printing operation, a poor image develops, and recording sheets are used in waste.